I've found box elders will make bitter/dark syrup when they're done. It'll taste fine at first, then it'll hit you. Boil small amounts down in a saucepan or similar and test it. You'll know when it's done The sap may seem the same as earlier in the year and be perfectly clear.
42.82N
2015 - Small operation. 25 buckets. One excited 5 year old and one 35 year old that feels 5 again.
2016 - One year older. New Homemade 2x4 Arch, Smoky Lake Pan and looking at 52 maples, 17 box elders and 2 walnut trees.
2017 - Shurflo 4008 hooked to 42 stingy silver maples and a few Norways. A couple buckets on sugars and Norways. 10 box elders.
2018 - ...a few more taps.
2019 - ...more taps on 3/16 gravity. This spiral is heading downward in a hurry.
2020 - 4x400 RO - RB20 (uh-oh!)